The present invention relates to a power transfer system for a four-wheel drive motor vehicle and, in particular, to a part-time transfer case having means for establishing an angular driving connection between the transfer case input and the output of a multi-speed transmission.
In view of the increased popularity of four-wheel drive vehicles, a plethora of power transfer systems are currently utilized for distributing tractive power (i.e., drive torque) to all four wheels of the motor vehicle. Conventionally, most four-wheel drive power transfer systems include a transfer case having an input shaft driven by the transmission output shaft, a rear output shaft driven by the input shaft and which is interconnected via a rear prop shaft to a rear axle assembly for driving the rear wheels, a front output shaft interconnected via a front prop shaft to a front axle assembly for driving the front wheels, and a torque transfer arrangement for continuously or selectively transferring drive torque from the rear output shaft to the front output shaft.
In "part-time" four-wheel drive systems, the transfer case is equipped with a shift mechanism which permits the vehicle operator to selectively couple and de-couple the front and rear output shafts for shifting the vehicle between a two-wheel drive mode and a four-wheel drive mode. An example of a part-time transfer case is shown and disclosed in commonly-owned U.S. Pat. No. 5,159,847 to Williams et al. In "full-time" four-wheel drive systems, the transfer case is equipped with an interaxle differential for continuously dividing drive torque between the front and rear output shafts while permitting speed differentiation therebetween. Commonly-owned U.S. Pat. No. 4,677,873 to Eastman et al. discloses an exemplary full-time transfer case. In order to prevent traction loss due to excessive wheel slip, most full-time transfer cases are also equipped with a "slip limiting" device for selectively or automatically locking the interaxle differential to limit or prevent speed differentiation in response to the wheel slip. Commonly-owned U.S. Pat. Nos. 5,078,660 and 5,106,351 to Williams et al. disclose exemplary full-time transfer cases equipped with a viscous coupling for limiting slip across the interaxle differential.
Alternatively, "on-demand" power transfer systems are used for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Typically, a speed-sensitive torque transfer device is installed between the front and rear output shafts for progressively delivering torque to the front output shaft in response to increasing speed differential therebetween. Such torque transfer devices commonly include viscous couplings, geared coupling and the like. An exemplary geared torque transfer device is disclosed in commonly-owned U.S. Pat. No. 5,358,454 to Bowen et al. More recently, however, the torque transfer device used in on-demand four-wheel drive systems has been a modulatable clutch, the output of which is controlled electronically in response to various detected vehicle operating parameters. An example of an electronically-controlled on-demand power transfer system is disclosed in commonly-owned U.S. Pat. No. 5,363,938 to Wilson et al.
Transfer cases are commonly classified as being either of a single offset type or a double offset type. In single offset transfer cases, only one of the output shafts is offset from the rotational axis of the input shaft. In particular, several of the above-noted commonly owned patents illustrate a conventional arrangement wherein the rotary axis of the rear output shaft is commonly aligned with that of the input shaft while the rotary axis of the front output shaft is parallel to, yet offset from, the rotary axis of the input shaft. However, a problem commonly associated with this type of transfer case is that the rear prop shaft, due to its central alignment, takes up a substantial amount of the vehicle's underbody space which inhibits placement of an underbody spare tire and/or necessitates usage of a saddle-type gas tank. In double offset transfer cases, the front and rear output shafts are commonly aligned and are both offset from the rotary axis of the input shaft. Accordingly, such double offset transfer cases provide increased underbody space for purposes of locating a spare tire and/or installing a larger fuel tank.
A major design consideration in the development of four-wheel drive systems is to minimize the departure angles at the connections between the transfer case front and rear output shafts and their corresponding prop shafts. The departure angle is defined as the included angle between the rotary axis of the prop shaft and that of the transfer case output shaft. Traditionally, single cardon universal joints are used at each end of the prop shafts if the departure angle is less than about five degrees. If the departure angles exceed this threshold limit, then more expensive double cardon universal joints are required at the prop shaft/transfer case connection. As an alternative, U.S. Pat. Nos. 4,632,207 to Moore and 5,116,293 to Reuter disclose the use of a constant velocity (CV) joint at the front output of a single offset type of transfer case. An alternative arrangement is likewise shown in U.S. Pat. No. 4,688,447 to Dick. While such arrangements attempt to solve the problems associated with large departure angles at the front output, they do not address this concern at the rear output nor are the teachings applicable to double offset transfer cases.